1. Field of the Invention
The present invention generally relates to analog-to-digital (A/D) converters and, more particularly, to an architecture for an A/D converter system which combines high resolution with a high sampling rate.
2. Description of the Prior Art
In many applications, a signal to be sampled and converted to a digital number is extremely small compared to added noise or interference, and if the signal were to be digitized by a low resolution A/D converter, all trace of it would be lost. Thus, even though it may have been possible to extract the signal out of the noise by filtering or other signal processing, the signal would be irretrievably lost due to quantization. In these instances, there are only two alternatives; either the signal processing must be performed in the analog domain, or the resolution of the A/D conversion must be increased. In some instances, it is not practical to perform the signal processing in the analog domain, and the only alternative is to increase the resolution of the A/D conversion. This, however, may be unacceptable where real time operation is required.
A/D converters may be divided into two generic categories - high speed, low resolution A/D converters, such as the so-called flash A/D converter, and low speed, high resolution A/D converters, such as Sigma-Delta and dual slope A/D converters. It would be desirable if the advantages of both categories of A/D converters could be combined into a single A/D converter which could thereby provide both high sampling rates and high resolution.
One obstacle to simply combining the two types of converters is that fast, high resolution A/D converters would require either a pipelined architecture or an impractically large number of comparators. The pipelined architecture requires thresholds that have an absolute precision at the level of the least significant bit, and it is impractical to implement these thresholds with present day comparator circuits. Practical comparators have uncertain thresholds due to subtle process variations and environmental parameters such as temperature.
The low speed A/D converter circuits mentioned above do not have the aforementioned drawbacks associated with pipelined architecture. This is because they are inherently incremental in operation, which is also the reason for their slow speed of operation.